1. Field of the Invention
The present invention relates to a method for driving a power supply system. In particular, the present invention relates to a method for driving a power supply system to which electric power is supplied by a magnetic resonance method.
2. Description of the Related Art
A method called a magnetic resonance method attracts attention as a method for supplying electric power to an object (hereinafter, also referred to as a power receiving device) in a state where contact with a power supply source (hereinafter, also referred to as a power transmitting device) is not made (such a method is also referred to as contactless power supply, wireless power supply, or the like). The magnetic resonance method is a method for forming an energy propagation path by making resonance coils provided in a power transmitting device and a power receiving device magnetically resonate with each other. The magnetic resonance method has a longer power transmittable distance than other methods (e.g., an electromagnetic induction method and an electric field induction method). For example, Non Patent Document 1 discloses that in the magnetic resonance method, transmission efficiency is approximately 90% when the distance between a pair of resonance coils is 1 m and that the transmission efficiency is approximately 45% when the distance between the pair of resonance coils is 2 m.
Note that short distance between the pair of resonance coils does not mean high transmission efficiency in the magnetic resonance method. Further, the transmission efficiency is changed by various factors such as the self resonant frequency of the resonance coil and the frequency of high-frequency voltage induced by the resonance coil. Therefore, when electric power is supplied by the magnetic resonance method, these factors are preferably optimized. For example, Patent Document 1 discloses a method for driving a power supply system in which the frequency of high-frequency voltage output from a high frequency power source is changed in accordance with an estimated distance between a pair of resonance coils.